FIG. 6 shows a schematic view of composition of lens structure and light source in a convention smart phone. A flashlight lens 61 of a conventional smart phone is Fresnel lens, with LED as light source 62. Fresnel lens has the capability of guiding light and collecting light, as well as thin, light, able to plasticized and low cost; therefore, the Fresnel lens is often used in flashlight. The shape of Fresnel lens is circular, flat and thin, with one surface acting as incident side 611 being ring-shaped jagged surface and the other surface being a flat light-emitting surface 612. When used with light source 62, jagged incident surface 611 faces the light source 62. The light source 62 can be a single LED or a plurality of LED arranged as an array.
When LED light source 62 emits light, the light is reflected by circular lens 61 and the luminance will be redistributed and projected onto the surface of the photography target object. The projected area is shaped as circular. The ideal situation is the circular area show uniform luminance theoretically. However, the actual distribution usually appears that the luminance in outer area is usually lower than the inner area. An alternative is to waste some light to non-effective outer area to obtain a more uniform light distribution. Also, as the captured image of the mobile phone usually has a rectangular effective area, the luminance in the area between the circular projected area and the rectangular area usually cannot be sensed by the light-sensing element.
Therefore, the it is desirable to devise a lens structure able to refract inwards the light of the outer are of circular projected area to address the above drawbacks of the conventional technique.